The most commonly employed photographic elements are those which contain a radiation sensitive silver halide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation sensitive silver halide microcrystals, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
Recently the photographic art has turned its attention to high aspect ratio tabular grain emulsions, herein defined as those in which tabular grains having an aspect ratio greater than 8:1 account for greater than 50 percent of the total grain projected area. The aspect ratio of the grains is determined by dividing the grain thickness by the grain diameter. The term grain diameter as used herein is its equivalent circular diameter--that is, the diameter of a circle having an area equal to the projected area of the grain. Grain dimensions can be determined from known techniques of microscopy. Tabular grain emulsions can offer a wide variety of advantages, including reduced silver coverages, thinner emulsion layers, increased image sharpness, more rapid developability and fixing, higher blue and minus blue speed separations, higher covering power, improved speed-granularity relationships, reduced crossover, less reduction of covering power with full forehardening, as well as advantages in image transfer. Research Disclosure, Vol. 225, January 1983, Item 22534, is considered representative of these teachings.
In almost every instance, the advantages of high aspect ratio tabular grain emulsions are enhanced by limiting the thickness of the tabular grains. High aspect ratio tabular grain silver chlorobromide emulsions having tabular grain thicknesses well below 0.3 .mu.m have been formed, and corresponding silver bromoiodide emulsions have been recently produced.
One possible drawback to tabular shaped grains is that they lie parallel when coated on a photographic paper or film support. Consequently, it is conceivable that overlapping layers could inhibit, to some degree, the free flow of developer solution.
By incorporating holes into the tabular grains, developer solution could be made to pass through the holes, resulting in more uniform development.
U.S. Pat. No. 4,713,323 to Maskasky discloses a process for preparing tabular grain emulsions. Although it does not appear to be a purpose of this patent, and therefore is incidental, FIG. 3 of Maskasky shows several grains having holes therein. However, the percentage of total grains having holes in this figure is very small.
U.S. Pat. No. 5,045,443 to Urabe discloses tabular silver halide grains wherein at least 30 percent of these grains have an indentation or space in their central portion. In the process disclosed by Urabe, the halogen composition of the grain is arranged so that the solubility of the center of the grain is higher than that of the surrounding portion. The central portion is then dissolved using a conventional silver halide solvent such as thiocyanate, leaving a centrally located hole. To make AgClBr grains, for example, Urabe teaches producing a grain in which the central portion is AgCl and the outer portion is AgClBr. The central AgCl portion is then dissolved using conventional silver halide solvents, leaving an AgClBr grain with a centrally located hole. The conventional ripening agents and fixing type solvents used to dissolve the more soluble halide portion of include, for example, thiocyanate, ammonia, thioether, and thiourea.